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United States Patent |
5,223,898
|
Fujii
,   et al.
|
June 29, 1993
|
Developing apparatus with the following roller closer to the drum than
the first roller
Abstract
A developing apparatus is used for developing a photoconductive drum in a
copying machine. The developing apparatus comprises a following developing
roller for developing said photoconductive drum, an opposing developing
roller for developing a photoconductive drum before the following
developing roller, a supply mechanism for supplying developer to the
developing rollers, a developer-quantity-regulator plate disposed between
the following developing roller and the supply mechanism, and a limiting
plate for limiting the length of the magnetic brush formed on the
following developing roller. The distance between the photoconductive drum
and the opposing developing roller is larger than the distance between the
photoconductive drum and the following developing roller. The regulator
plate is disposed apart from the developing rollers at a distance longer
than the length of their magnetic brushes. The distance between the
following developing roller and the limiting plate is within the length of
the magnetic brush formed when the following developing roller starts
rotating and the length of the magnetic brush formed when the following
developing roller rotates at a constant speed.
Inventors:
|
Fujii; Jiro (Kadoma, JP);
Tamura; Yasuaki (Higashiosaka, JP);
Kondo; Kazuhisa (Tokyo, JP);
Shono; Hidekazu (Kawanishi, JP)
|
Assignee:
|
Mita Industrial Co., Ltd. (Osaka, JP)
|
Appl. No.:
|
540640 |
Filed:
|
June 19, 1990 |
Foreign Application Priority Data
| Jun 29, 1989[JP] | 1-167498 |
| Jun 29, 1989[JP] | 1-167499 |
| Jun 29, 1989[JP] | 1-167500 |
Current U.S. Class: |
399/254; 399/279; 399/343 |
Intern'l Class: |
G03G 015/06 |
Field of Search: |
355/208,245,246,251,252,253,259,260
118/656,657,658,653
|
References Cited
U.S. Patent Documents
3640248 | Feb., 1972 | Nielander | 118/637.
|
4098228 | Jul., 1978 | Ruckdeschal et al. | 118/658.
|
4261290 | Apr., 1981 | Yamashita et al. | 118/658.
|
4394429 | Jul., 1983 | Hays | 118/653.
|
4580888 | Apr., 1986 | Thompson et al. | 355/3.
|
Foreign Patent Documents |
0109374 | Aug., 1981 | JP | 118/658.
|
0174863 | Oct., 1984 | JP | 355/253.
|
2037620 | Jul., 1980 | GB | 118/657.
|
2141048A | Dec., 1984 | GB.
| |
Primary Examiner: Grimley; A. T.
Assistant Examiner: Horgan; Christopher
Attorney, Agent or Firm: Beveridge, DeGrandi, Weilacher & Young
Claims
What is claimed is:
1. An apparatus for developing a latent image on a photoconductive drum of
an image forming apparatus comprising:
a following developing roller for developing said latent image;
an opposing developing roller for developing said latent image located
before said following developing roller, said opposing developing roller
being located at a distance from said photoconductive drum which is larger
than the distance between said following developing roller and said
photoconductive drum; and
supply means for supplying developer to said developing rollers.
2. An apparatus according to claim 1, wherein the ratio of the distance
between said opposing developing roller and said photoconductive drum to
the distance between said following developing roller and said
photoconductive drum ranges from 1.1 to 1.5.
3. An apparatus according to claim 2, wherein said ratio is 1.3.
4. An apparatus according to claim 3, wherein the distance between said
opposing developing roller and said photoconductive drum is 1.3 mm, and
the distance between said following developing roller and said
photoconductive drum is 1.0 mm.
5. An apparatus according to claim 1, wherein said opposing developing
roller is positioned above said following developing roller.
6. An apparatus according to claim 5, further comprising a housing and a
regulator plate for controlling the quantity of developer flow disposed
between said following developing roller and said supply means and apart
from said housing to provide a path through which the developer passes,
and apart from a developer layer formed on said following developing
roller, said regulator plate being at a distance from said following
developing roller which is larger than said path.
7. An apparatus according to claim 6, further comprising a limiting plate
for limiting the thickness of said developer layer on said following
developing roller, said limiting plate being apart from said developing
roller at a distance which is less than the thickness of said developer
layer formed when said developer layer starts rotating and more than the
thickness of said developer layer formed when said following developing
roller rotates at a constant speed.
8. An apparatus according to claim 7, wherein said limiting plate is
disposed above a stationary portion of developer which is under said
following developing roller.
9. An apparatus according to claim 8 further comprising a separating plate
having said limiting plate at its lower end and disposed to be opposite
said photoconductive drum on the other side of said developing rollers.
10. An apparatus according to claim 9, wherein said supply means comprises
a toner hopper for supplying toner, sloping plates for mixing toner having
a plurality of perforations, disposed below said toner hopper, and a
mixing and transferring roller disposed below said sloping plates.
11. An apparatus for processing an image comprising:
an optical system for obtaining image information from an original;
a photoconductive drum for forming an electrostatic latent image
corresponding to the obtained image information;
an image processing part surrounding said photoconductive drum, which
includes a developing apparatus having a following developing roller for
developing the latent image, an opposing developing roller for developing
the latent image located before said following developing roller, and
supply means for supplying developer to said developing rollers; wherein
the distance between said opposing developing roller and said
photoconductive drum is larger than the distance between said following
developing roller and said photoconductive drum;
paper supply means for supplying a sheet of paper to said image processing
part; and
a fixing unit for fixing the image onto said sheet of paper from said image
processing part.
12. An apparatus for developing a latent image on a photoconductive drum of
an image forming apparatus comprising:
developing rolling means adjacent to said photoconductive drum;
supply means for supplying developer to said developing rolling means;
a regulator plate for controlling the quantity of developer flow located
apart from the developer layer formed on said developing rolling means and
between said developing rolling means and said supply means; and
a housing containing said developing rolling means, said supply means and
said regulator plate, wherein said regulator plate is located apart from
said housing to define a path through which the developer passes, and said
regulator plate is apart from said developing rolling means at a distance
larger than said path.
13. An apparatus according to claim 12, wherein the ratio of the distance
between said regulator plate and said developing rolling means to the
distance between said regulator plate and said housing ranges from 1.5 to
2.0.
14. An apparatus according to claim 13, wherein said ratio ranges from 1.7
to 1.8.
15. An apparatus according to claim 14, wherein the distance between said
regulator plate and said housing is 7 mm, and the distance between said
regulator plate and said developing rolling means is 12 mm.
16. An apparatus according to claim 15 further comprising a limiting plate
for limiting the thickness of the developer layer formed on said
developing rolling means, disposed apart from said developing rolling
means in a distance shorter than the thickness of the developer layer
formed when said developing rolling means starts rotating and longer than
the thickness of the developer layer formed when said developing rolling
means rotates at a constant speed.
17. An apparatus according to claim 16, wherein
said developing rolling means comprises a following developing roller for
developing said photoconductive drum, and an opposing developing roller
for developing said photoconductive drum before said following developing
roller; and
said supply means comprises a toner hopper for supplying toner, a sloping
plate having a plurality of perforations disposed below said toner hopper,
for mixing toner, and a mixing and transferring roller disposed below said
sloping plate.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a developing apparatus for use in an image
forming apparatus. More specifically, it relates to a developing apparatus
for developing a photoconductive drum of an image forming apparatus.
(1) Japanese Patent Publication No. 49586/1984 discloses a developing
apparatus of an electrophotographic copying machine which comprises a
housing adjacent to a photoconductive drum, an opposing developing roller
and a following developing roller rotatably installed in the housing, and
a supply mechanism for supplying developer to the developing rollers. In
this developing apparatus, the distance between the photoconductive drum
and the opposing developing roller is smaller than that between the
photoconductive drum and the following developing roller.
Since the distance of the opposing developing roller to the photoconductive
drum is smaller than that of the following developing roller, the
conventional developing apparatus causes the phenomenon shown in FIG. 1.
In FIG. 1, number 50 designates a photoconductive drum, number 51
designates an opposing developing roller, and number 52 designates a
following developing roller. Arrows show the rotation directions of the
rollers. Developer 53 is indicated by small dots.
As used in this specification and shown in FIGS. 1 and 3, the term
"opposing developing roller" refers to a roller whose axis is turning in
the same direction as the axis of the photoconductive drum and is
positioned such that the surface of the opposing developing roller is
turning opposite to the direction which the photoconductive drum surface
is turning. The opposing developing roller is shown at reference number 51
in FIG. 1 and reference number 22 in FIG. 3. Likewise, in this
specification, the term "following developing roller" refers to a roller
whose axis is turning in the opposite direction as the axis of the
photoconductive drum and is positioned such that the surface of the
following developing roller is turning in a direction which follows the
direction which the photoconductive drum surface is turning. The following
developing roller is shown at reference number 52 in FIG. 1 and reference
number 23 in FIG. 3.
In FIG. 1, the developer 53 drawn up first by the following developing
roller 52 can pass through the gap between the rollers 51 and 52, and then
about half that amount is transferred to the opposing developing roller
51. The two divided portions of the developer 53 are transferred toward
the photoconductive drum 50 by the rotation of the rollers 51 and 52.
Since the photoconductive drum 50 rotates in the direction opposite to the
direction of developer 53 transferred by the opposing developing roller
51, there is a tendency to transfer the developer 53 from the opposing
developer 51 toward the following developing roller 52 by the rotation of
the photoconductive drum 50. Consequently, a portion of the developer 53
accumulates on the opposing developing roller 51, hanging like an icicle
(see part indicated P in FIG. 1), and such portion of the developer 53 is
reverted to the following roller 52. That causes an excess amount of the
developer 53 to be supplied to the following developer 52, whereby the
load on the following developing roller 52 is increased and copy quality
is lowered. According to experiments performed by the inventors of the
present invention, the phenomenon became more pronounced as the distance
between the opposing developing roller 51 and the photoconductive drum 50
was made narrower than that between the following developing roller 52 and
the photoconductive drum 50.
(2) United Kingdom Patent Application GB 2,141,048A discloses a developing
apparatus of an electrophotographic copying machine which comprises a
housing adjacent to a photoconductive drum, developing rollers rotatably
installed in the housing, and a regulator plate disposed in the part of
developer to be supplied to the developing rollers. In this developing
apparatus, the gap between the regulator plate and the lower developing
roller becomes filled with developer, because the gap the developer passes
is narrow. The developer which has passed the gap is transferred forward
the photoconductive drum by the developing rollers to develop the
electrostatic latent image on the photoconductive drum.
In this conventional developing apparatus, developer is deposited between
the regulator plate and the developing roller, wherein particles of the
developer become magnetically adhered to each other due to magnetization
by the developing roller. As a result, the developer becomes a hardened
mass in the gap between the regulator plate and the developing roller,
making rotation of the developing roller difficult. Consequently,
increased torque on the developing roller is required, due to the heavier
load on the developing roller.
Moreover, the gap between the regulator plate and the developing roller is
not narrow enough whereby it may act as a measure for measuring the amount
of the developer. The amount of developer that is drawn up by the
developing roller is largely self-determined. That is, the entire amount
of the developer drawn up by the developing roller by magnetic force can
pass the gap between the developing roller and the regulator plate. As the
developer is transported by the developing roller, it forms a magnetic
brush on the surface of the developing roller corresponding to the
magnetic force of the developing roller. Thus, the regulator plate of the
developing apparatus cannot act to limit the length of the magnetic brush,
whereby the load on and abrasion of the developing roller in the common
type of developing apparatus are reduced, and wherein the regulator plate
always controls the length of the magnetic brush by trimming the top
portion of the magnetic brush.
In this conventional developing apparatus, as described above, the length
of the magnetic brush is self-determined by means of the developing
roller, wherein the length of the magnetic brush is stable and constant as
long as the developing roller rotates at a constant speed. However, at the
start of the developing process, or when the developing roller begins to
rotate, the developing roller momentarily tends to carry a large amount of
developer due to its slow initial speed of rotation. Therefore, in the
conventional apparatus, a large excess amount of developer is supplied to
the photoconductive drum, causing an increase of scattered toner and a
sudden change in load.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a developing apparatus
which requires relatively low developing-roller torque.
It is another object of the present invention to provide a developing
apparatus which decreases the supplying of excess developer to a
developing roller caused by the rotation of the photoconductive drum, and
which thereby results in less disruption of an image and reduction in load
on the developing roller.
It is yet another object of the present invention to provide a developing
apparatus which impedes a large amount of excess developer form being
transferred by the developing rollers at the start of the developing
process wherein a large amount of excess developer causes increased toner
scattering and a sudden change in load.
(1) According to an aspect of the present invention, a developing apparatus
is an apparatus for developing a photoconductive drum. The developing
apparatus comprises a following developing roller for developing the
photoconductive drum, an opposing developing roller for developing the
photoconductive drum before the following developing roller, and a supply
mechanism for supplying developer to the developing rollers. The distance
between the photoconductive drum and the opposing developing roller is
longer than that between the photoconductive drum and the following
developing roller.
In this developing apparatus, the supply mechanism supplies the developer
to the developing rollers. The opposing developing roller first develops a
latent image on the photoconductive drum, and then the following
developing roller develops the image.
The opposing developing roller transfers the developer in the direction
opposite to the direction of the rotation of the photoconductive drum.
Accordingly, the photoconductive drum tends to retrograde developer onto
the opposing developing roller. However, in this developing apparatus, the
distance between the photoconductive drum and the opposing developing
roller is longer than that between the photoconductive drum and the
following developing roller, so that the tendency to retrograde is
diminished, and the developer on the opposing developing roller can be
properly transferred in the direction of the rotation of the opposing
developing roller. This solves the problem wherein a large amount of
developer on the opposing developing roller is reverted to the following
developing roller by the rotation of the photoconductive drum.
(2) According to another aspect of the present invention, a developing
apparatus is an apparatus for developing a photoconductive drum. This
developing apparatus comprises developing rollers adjacent to the
photoconductive drum, a supply mechanism for supplying developer to the
developing rollers, and a regulator plate for controlling the quantity of
developer flow toward the developing rollers, disposed between the
developing rollers and the supply mechanism. The regulator plate is
located apart from the developer layer formed on the developing rollers.
In this developing apparatus, when the supply mechanism supplies the
developer to the developing rollers, the regulator plate controls the
quantity of the developer flow (to the developing rollers). The developing
rollers rotate to develop the photoconductive drum with the received
developer.
The regulator plate is located apart from the developer layer formed on the
developing rollers, that is, the magnetic force of the developing roller
does not substantially reach the regulator plate. Accordingly, since there
is a gap between the regulator plate and the developer layer formed on the
developing rollers, the space between the developing rollers and the
regulator plate is not filled with the developer carried by the developing
rollers. Therefore, the regulator plate is not an obstacle to the rotation
of the developing rollers, whereby the present invention lowers the
required driving force of the developing roller.
(3) According to yet another aspect of the present invention, a developing
apparatus is an apparatus for developing a photoconductive drum. This
developing apparatus comprises developing rollers adjacent to a
photoconductive drum, a supply mechanism for supplying developer to the
developing rollers, and a limiting plate for limiting the length of the
magnetic brush formed on one of the developing rollers. The distance
between the developing roller and the limiting plate is defined to be
shorter than the length of the magnetic brush formed when the developing
roller starts to rotate and greater than the length of the magnetic brush
formed when the developing roller rotates at a constant speed.
In this developing apparatus, the supply mechanism supplies the developer
to the developing rollers. The supplied developer forms the magnetic brush
on the developing rollers.
When the developing process starts, a large amount of developer is
momentarily carried on the developing roller to form a magnetic brush of
excess length, due to an initially slow speed of rotation. This magnetic
brush is limited in length by the limiting plate, whereby the magnetic
brush is shortened. This results in less scattered toner and less sudden
change in load at the start of the developing apparatus.
Subsequently, the developing roller rotates at a stable speed, so that the
magnetic brush is stable in a short length on the developing roller. Under
this condition, the limiting plate does not increase the load of the
developing roller, because the limiting plate no longer limits the length
of the magnetic brush.
These and other objects and advantages of the present invention will be
more fully apparent from following the detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional side view showing a part of a conventional apparatus;
FIG. 2 is a sectional, schematic view showing a copying machine having a
developing apparatus according to the present invention;
FIG. 3 is a sectional view showing the body of the developing apparatus;
and
FIGS. 4 and 5 are sectional views showing part of a limiting plate.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 is a sectional view showing a copying machine which includes an
embodiment according to the present invention. In the figure, a copying
machine body 1 has a contact glass 2 in the upper surface and an original
holder 3 thereon which can be opened. On the right side of the machine
body 1 in the figure, a pair of detachable paper cassette cases 5 and 6
are attached. On the left side of the machine body 1, a copy tray 7 is
attached wherein copied paper is received.
In the machine body 1, an optical exposure system 8 for obtaining
information from an original is located in the upper portion. The exposure
system 8 consists of a light source, mirrors and so on. Disposed in the
central part of the machine body 1 is a photoconductive drum 9 on which an
electrostatic latent image is formed. Surrounding the photoconductive drum
9, there are a corona-generating main charger 10 for charging the
photoconductive drum 9 with a predetermined level of electric charge, a
developing apparatus 11 as an embodiment according to the present
invention, a corona-generating transfer unit 12 for transferring a toner
image to paper, a corona-generating detach unit 13 for detaching paper
from the photoconductive drum 9, and a cleaning unit 14 for removing toner
form the photoconductive drum 9, in that order.
On the attached ends of the paper cassettes 5 and 6, a pair of feeding
rollers 16a and 16b are disposed. A paper feeding unit 17, for feeding
material paper from the cassettes 5 and 6 to the image-forming part
including the photoconductive drum 9, is located beyond the rollers 16a
and 16b with respect to the paper flow direction (the right-to-left
direction in FIG. 2). A transfer unit 18 is provided in a space beyond the
image forming part with respect to the paper flow direction. A fixing unit
20 for fixing a transferred image onto the fed paper is disposed between
the transfer unit 18 and ejecting rollers 19.
The developing apparatus 11 consists of an apparatus body 21 and a toner
hopper 35 attached onto the body 21. The toner hopper 35 contains toner to
be supplied to the body 21.
Referring to FIG. 3: the apparatus body 21 is located adjacent to the
photoconductive drum 9, which rotates in the direction of arrow A. The
apparatus body 21 has an opposing developing roller 22 and a following
developing roller 23 which are along side each other in parallel and face
the photoconductive drum 9. The rollers 22 and 23 are disposed rotatably
in directions opposite to each other in a housing 24, wherein the roller
22 is above the rollers 23. Both rollers 22 and 23 have a plurality of
magnets (not shown) inside. The direction of the rotation of the opposing
developing roller 22, shown by arrow B, is the same as that of the
photoconductive drum 9, so that with respect to their facing portions, the
surface of the roller 22 moves in the direction opposite to that of the
photoconductive drum 9. The direction of the rotation of the following
developing roller 23, shown by arrow C, is opposite to that of the
photoconductive drum 9, so that in their facing portions, the surface of
the roller 23 moves in the same direction as that of the photoconductive
drum 9.
Both rollers 22 and 23 are apart from each other so that developer can be
transferred between the rollers 22 and 23 by the magnetic force of the
inner magnets. The distance between the photoconductive drum 9 and the
opposing developing roller 22 is larger than that between the
photoconductive drum 9 and the following developing roller 23. The ratio
of the distance between the photoconductive drum 9 and the opposing
developing roller 22 to the distance between the photoconductive drum 9
and the following developing roller 23 preferably ranges from 1.1 to 1.5,
with the most preferable being 1.3. For example, the distance between the
photoconductive drum 9 and the opposing developing roller 22 may be 1.3
mm, and the distance between the photoconductive drum 9 and the following
developing roller 23 may be 1.0 mm.
Disposed in the housing below the toner hopper 35 are sloping plates 25
down along which toner dropping from the toner hopper 35 moves. The
sloping plates 25 consists of three plates located apart from each other
in parallel. The plates 25 have a number of perforations (not shown) to
allow the toner to drop, whereby the toner is mixed with carrier to form
developer as the toner moves down along the sloping plates 25.
Below the sloping plates 25, a mixing and transferring roller 26 is
provided to mix the developer for inclusion of toner and for transferring
the developer to the rollers 22 and 23. The mixing and transferring roller
26 is driven to rotate in the counterclockwise direction, shown by arrow
E. The roller 26 also has a plurality of magnets (not shown) inside.
Between the mixing and transferring roller 26 and the following developing
roller 23, a developer transferring path 28 is provided which is formed
wherein the housing 24 incorporates a guide member 27. Above the middle
part of the developer transferring path 28, a regulator plate 29 for
controlling the quantity of toner flow is located in a vertical extension.
There is a gap 30 between the regulator plate 29 and the guide member 27
to control the quantity of toner flow. The gap 30 may be, for example, 7
mm in height. The regulator plate 29 is located apart from the following
developer roller 23 at some distance. The distance between the regulator
plate 29 and the following developing roller 23 may be, for example, 12
mm. The ratio of the distance between the regulator plate 29 and the
following developing roller 23 to the distance between the regulator plate
29 and the guide member 27 preferably ranges from 1.5 to 2.0, and more
preferably from 1.7 to 1.8. Although the magnet in the following
developing roller 23 nearest to the regulator plate 29 is 530G in magnetic
induction, but no substantial magnetic force from the magnet reaches the
regulator plate 29. Consequently, the regulator plate 29 is located apart
from the following developing roller 23 at a length longer than the
thickness of the developer layer formed on the following developing roller
23, or longer than the length of the magnetic brush. That is, there exists
a space between the regulator plate 29 and the developer layer--or the
magnet brush--on the following developing roller 23.
A removing roller 31 is provided in a region beyond (with respect to the
developer flow) the opposing developing roller 22, and the top ends of the
sloping plates 25 are positioned a little lower than the removing roller
31. The removing roller 31 is driven to rotate in the direction shown by
arrow D, which is the same as that of the opposing developing roller 22,
and contains a plurality of magnets (not shown).
A separating plate 32 is provided adjacent to the removing roller 31 and
the opposing developing roller 22 so that the separating plate 32 is
opposite the photoconductive drum 9 (through the roller 22). The lower end
of the separating plate 32 bends toward the following developing roller 23
and forms a limiting plate 33. The limiting plate 33 has its end located
apart from the following developing roller 23 a little further than the
thickness of the developer layer, or the length of the magnetic brush,
formed on the following developing roller 23 when the roller 23 rotates at
a constant speed. For example, the layer of the developer may be 1.8 mm in
thickness, and the gap between the following developing roller 23 and the
limiting plate 33 may be 2 mm in length. The ratio of the distance between
the following developing roller 23 and the limiting plate 33 to the
thickness of the constant layer of the developer formed on the following
developing roller 23 preferably ranges from 1.0 to 1.6, more preferably
from 1.0 to 1.3, and is best at 1.1. In other words, the distance between
the limiting plate 33 and the constant layer formed on the following
developing roller 23 is preferably in the range from 0.0 to 1.2 mm, more
preferably from 0.0 to 0.6 mm, and is best at 0.2 mm. The limiting plate
33 is positioned above the stationary portion of the developer under the
following developing roller 23 wherein a relatively large amount of
developer accumulates (as indicated by a concentration of small dots in
the region of the limiting plate 33 in FIG. 3). Thus, the end of the
limiting plate 33 faces only onto the magnetic brush formed on the
following developing roller 23.
The operation of the above embodiment will be described.
Wherein a copy is to be obtained, first the original holder 3 is opened,
next an original is placed on the contact glass 2, and then the original
holder 3 is closed.
When the start button (not shown) is pushed, the exposure system 8 moves to
scan the original on the contact glass 2, whereby a latent image in
correspondence with the original is recorded on the photoconductive drum
9.
Meanwhile, in the developing apparatus 11, each of the rollers 22, 23, 26
and 31 is driven to rotate as shown by the directions of the arrows. By
this rotation, the developer is circulated in the body 21 of the
developing apparatus 11 along the course shown by the small arrows. As
circulation starts, the following developing roller 23 draws up developer
from the stationary portion below the following developer roller 23,
whereby a magnetic brush of the developer is formed on the following
developing roller 23. However, the roller 23 initially must rotate slowly
as it attains constant speed, whereupon a large excess amount of developer
is brought onto the following developing roller 23. That is, a longer
magnetic brush is temporarily formed. As shown in FIG. 4, the longer
portion of the magnetic brush is brought to the limiting plate 33 by the
rotation of the following developing roller 23, and the limiting plate 33
restrains the longer part of the magnetic brush. The top portion of the
magnetic brush is removed, or trimmed, by the limiting plate 33, whereby
its length is brought to the constant length, after the brush passes the
limiting plate 33. Thus, even during the starting period in the developing
apparatus 11, no large amount of excess developer can be supplied to the
photoconductive drum 9, whereby undesirable developer scattering and
sudden load change due to supplying a large amount of developer are
suppressed.
After the initial rise time in the developing process, the following
developing roller 23 rotates at constant speed, so that a magnetic brush
having a stable length is formed on the roller 23, as shown in FIG. 5. In
this instance, the magnetic brush is about 1.8 mm in length, so that the
magnetic brush remains apart from the end of the limiting plate 33 by a
small gap. Therefore, the limiting plate 33 cannot take effect while the
following developing roller 23 rotates at a constant speed, thereby the
limiting late 33 does not create a load on the following developing roller
23.
The photoconductive drum 9 having an electrostatic latent image rotates in
the direction of the arrow A in FIG. 3 to convey the latent image to the
developing rollers 22 and 23 having the stable-length magnetic brushes
thereon. Meanwhile, in the body 21 of the developing apparatus 11, the
developer circulates along the course indicated by the small arrows. That
is, the developer moves down along the sloping plates 25 to be mixed.
Further along, the developer reaches the mixing and transferring roller 26
along the bottom surface of the housing 24. Then, the developer is mixed
by the rotation of the roller 26 to become agitated. The agitated
developer is transferred by the roller 26 along the developer transferring
path 28 to the following developing roller 23. Since the regulator plate
29 is positioned above the middle part of the transferring path 28, the
quantity of the developer flow is properly limited, or measured by the gap
30 between the guide plate 27 and the regulator plate 29. Therefore, a
predetermined amount of the developer from the mixing and transferring
roller 26 side is exactly transferred to the following developing roller
23.
The following developing roller 23 rotates in the direction shown by the
arrow C. This rotation, and the magnets installed in the following
developing roller 23, form a magnetic brush from the developer on the
roller 23. Since the regulator plate 29 is located apart from the
developer layer formed on the following developing roller 23, there exists
some distance between the magnetic brush and the regulator plate 29.
Therefore, the rotation of the following developing roller 23 is not
disturbed by developer accumulating in the space between the following
developing roller 23 and the regulator plate 29. As a result, excess load
due to the developer movement cannot be put on the following developing
roller 23, thereby only a small force of rotation of the following
developing roller 23 is required.
The developer transferred by the following developing roller 23 is moved
through the gap between the following developing roller 23 and the
limiting plate 33 toward the photoconductive drum 9. When the following
developing roller 23 rotates initially, the length of magnetic brush is
not stable, consequently the magnetic brush may occasionally be longer
than that wherein the roller 23 is rotating at a constant speed. In this
interval, the limiting plate 33 functions, whereby the length of the
magnetic brush is made constant. After the initial rise time, the
following developing roller 23 rotates at constant speed, and the limiting
plate 33 becomes inoperative. When the following developing roller 23
rotates at constant speed, the developer carried by the following
developing roller 23 is entirely transferred under the limiting plate 33
toward the photoconductive drum 9, due to the fact that the magnetic brush
is shorter than the distance between the roller 23 and the limiting plate
33.
Part of the developer carried by the following developing roller 23 is
transferred to the opposing developing roller 22 to form a magnetic brush
on the opposing developing roller 22. The magnetic brushes on the
following developing roller 23 and the opposing developing roller 22 are
conveyed to the photoconductive drum 9. Then, the developer reaches the
developing position wherein it faces the photoconductive drum 9 and
develops the electrostatic latent image onto the photoconductive drum 9.
In this stage, the developer on the opposing developing roller 22 is moved
in the direction opposite to the moving direction of the surface of the
photoconductive drum 9, whereas the developer on the following developing
roller 23 is moved in the direction following the moving direction of the
photoconductive drum 9.
When the developer on the opposing developing roller 22 is transferred in
the direction opposite to the moving direction of the surface of the
photoconductive drum 9, the developer undergoes force from the
photoconductive drum 9 whereby the developer is drawn back by the rotation
of the photoconductive drum 9. However, since the distance between the
photoconductive drum 9 and the opposing developing roller 22 is larger
than that between the photoconductive drum 9 and the opposing developing
roller 23 in this embodiment, passage developer through the gap between
the photoconductive drum 9 and the opposing developing roller 22 is
facilitated. Therefore, developer on the opposing developing roller 22 is
not reverted to the following developing roller 23 by the rotation of the
photoconductive drum 9, but is properly transferred toward the removing
roller 31 by the rotation of the opposing developing roller 22. Thus, in
the above embodiment, the conventional problem as shown in FIG. 1, namely
the phenomenon indicated by the arrow P, does not occur.
The developer on the following developing roller 23 is moved in
synchronization with the rotation of the photoconductive drum 9 to develop
the electrostatic latent image. At this stage, developer on the opposing
developing roller 22 is not reverted to the following developing roller 23
by the rotation of the photoconductive drum 9, so that no large amount of
excess developer is supplied to the following developing roller 23, and
the development is optimally carried out. Accordingly, wherein an
excessive supply of developer reverted from the opposing developing roller
22 induces load increase on the following developing roller 23, which in
turn may disarrange a toner image on the photoconductive drum 9, herein it
does not occur.
The developer having passed through the developing position on the
following developing roller 23 is transferred under the roller 23 toward
the regulator plate 29. Meanwhile, the developer transferred from the
opposing developing roller 22 to the removing roller 31 is further
transferred to the sloping plates 25. If toner concentration in the
developer is lower than a predetermined level, the toner hopper 35
supplies new toner onto the sloping plates 25.
As the photoconductive drum 9 developed by the developing apparatus 11
continues to rotate, the developed image on the photoconductive drum 9 is
moved to the position of the transfer charger 12 and transferred by the
transfer charger 12 to a sheet of material paper from either paper
cassette 5 or 6. After transferring the image, the surface of the
photoconductive drum 9 is cleaned by the cleaner 14. Meanwhile, the paper
having the toner image is supplied to the fixing unit 20 by the transfer
unit 18, and then the fixed paper is discharged to the copy tray 7.
Although the above embodiment has a pair of developing rollers 22 and 23,
the regulator plate 29 and the limiting plate 33 can be employed also in a
developing apparatus having one developing roller, or several developing
rollers.
Experiments
The distance between the photoconductive drum 9 and the opposing developing
rollers 22 was varied, while the distance between the photoconductive drum
9 and the following developing roller 23 was fixed. The experiment was
performed with a developing apparatus which is the same as that in the
above description.
The distance X between the photoconductive drum 9 and the following
developing roller 23 was fixed at 1.0 mm, and the distance Y between the
photoconductive drum 9 and the opposing developing roller 22 was varied as
shown in Table 1. In the experiment, the main magnetic pole within the
opposing developing roller 22 is set at an angle of -12.degree. with
respect to the line between the centers of the photoconductive drum 9 and
the opposing developing roller 22.
The results are shown in Table 1. Whether a result was normal or abnormal
was decided contingent upon whether the developer formed any "icicle"
portion, as indicated by the arrow P in FIG. 1.
TABLE 1
______________________________________
Y 0.9 mm 1.0 mm 1.1 mm 1.2 mm 1.3 mm
______________________________________
abnormal?
Yes Yes No No No
______________________________________
As shown in Table 1, the developer performed abnormally in the range of
X.ltoreq.Y, but normally in the range of X>Y.
Next, the distance between the following developing roller 23 and the
limiting plate 33 was varied. The distances are set at 1.5 mm, 2.0 mm, 2.5
mm and 3.0 mm, and the quality of the copied images were then checked. The
experiment was performed with a magnetic brush of the following developing
roller 23 whereof length was about 1.8 mm during constant rotation.
In the case of 1.5 mm, the developed image was disarranged by toner
slippage. In the cases of 2.0 mm, 2.5 mm and 3.0 mm, sharp images were
obtained on copies. However, in the cases of 2.5 mm and 3.0 mm, a large
amount of toner was scattered from the developing apparatus 11.
Various details of the invention may be changed without departing from its
spirit nor its scope. Furthermore, the foregoing description of the
embodiments according to the present invention is provided for the purpose
of illustration only, and not for the purpose of limiting the invention as
defined by the appended claims and their equivalents.
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